One issue that should be considered to start a wind turbine generator is to avoid undesired influence on the utility grid (for example, to reduction in the voltage of the utility grid) when the wind turbine generator is interconnected to the utility grid to be brought online. As a technique for preventing undesired influence on the utility grid, there is known a technique using a soft starter, for example. Such a technique is disclosed in, for example, Japanese Unexamined Patent Publication JP 2005-39924 A.
One parameter that should be considered in starting the wind turbine generator is the generator speed (or the wind turbine speed). It is an effective approach for avoiding undesired influence on the utility grid to interconnect the wind turbine generator to the utility grid in a state in which the generator speed is controlled to an appropriate rotational speed (referred to as “interconnection speed”).
It is necessary to set the pitch angle of the wind turbine blades to the fine pitch angle so as to raise the generator speed of the generator up to the interconnection speed. Here, a method is adopted in which the pitch angle is gradually increased from the feather pitch angle to the fine pitch angle, because of the necessity to gradually increase the wind turbine speed. One typical procedure is as follows: Referring to FIG. 5, the pitch angle is first set to a predetermined waiting pitch angle θwait. In this state, when the wind turbine speed is increased up to a predetermined rotational speed (waiting speed), the pitch angle is increased up to the fine pitch angle θfine at a predetermined pitch rate. Thereafter, the wind turbine generator is interconnected to the utility grid when the wind turbine speed reaches the interconnection speed. Although the wind turbine generator is interconnected to the utility grid after the pitch angle reaches the fine pitch angle θfine in the example shown in FIG. 5, the wind turbine speed may reach the interconnection speed before the pitch angle reaches the fine pitch angle θfine.
When a delay occurs to the follow-up of the rotational speed, however, such method may result in that the pitch angle is set to an excessive attack angle for the wind speed of the wind flowing into the wind turbine, causing stall of the wind turbine blades. A delay may occur to the follow-up of the rotational speed, for example, when the wind speed is reduced after the pitch angle starts being increased from the waiting speed or when the blades suffer from deterioration of the aerodynamic performance, weight increase or occurrence of unbalance, due to icing thereon. When the wind turbine blades are placed into the stall state, this disables the increase in the rotational speed, causing a start-up failure. In this case, it is required to start the start-up procedure over again, which disadvantageously causes a problem in the operation.